The processes used to manufacture amines tend to produce mixtures of these compounds. These mixtures can be resolved in a variety of ways. For example, distillation can be used to fractionate these mixtures. It is well known that amines such as polyethylene amines become discolored during manufacture and/or during storage in air. The mixtures often are yellow, yellow brown, or dark brown in color. It is believed in the industry that the discoloration is attributable to impurities in the product mixtures. These impurities may be by-products of the process of making the amines. Usually, the commercial value of strongly discolored amines is less than the commercial value of low color amines, often very significantly less. Accordingly, many processes have been devised to decolorize compositions including one or more of these amine compounds.
Strong mineral acids, e.g., those having a pKa of under about 1.0 at 25° C., include hydrogen halides such as HCl, HBr, and HF as well as sulfuric acid, nitric acid, combinations of these, and the like. Strong mineral acids have been used to decolor amines. Each of GB 1351050A, DE 2163516C3, and JP 1970-119902 describes a process of decolorizing polyethyleneamines by vacuum distillation in the presence of their hydrochlorides. The hydrochloride is obtained by adding hydrochloric acid to the amines.
U.S. Pat. No. 5,861,537 (1999) describes a process of decolorizing polyetheyleneamines by treating the amities with hydrogen halides, distilling TETA and lower boiling amines, then neutralizing the excess acid in the residual liquor by adding alkali and then distilling the high-boiling amines from the mixture.
Although hydrogen halides offer a high degree of decolorization, the use of hydrogen halides is quite problematic. Firstly, hydrogen halides tend to be highly corrosive, strong acids. These acids have a strong corrosive effect on equipment unless expensive, corrosion-resistant equipment is used. These acids require careful handling. Further, when reacted with compositions including one or more amines, these acids tend to react with the amines to form amine hydrohalide by-products. For example, if HCl is used, the amine hydrochlorides remain in the bottom of a distillation column when the amine-HCl mixture is distilled for recovering the decolorized amine. The amine hydrochloride must be either disposed of as a low value byproduct, neutralized by adding an aqueous alkali solution to liberate the amine, and/or the like. The byproduct salt and water then need to be separated from the amine product(s). The added HCl can lead to acid-catalyzed amine decomposition if too much acid is added and/or too much heat is added. As little as 1% by weight HCl in amine can decrease the amine decomposition temperature by 50° C., for instance. Therefore, HCl concentration in the amine composition must be tightly controlled. Further, the process disclosed by U.S. Pat. No. 5,861,537 requires heating the mixture to high temperatures (150° C. to 240° C.) for effective decolorization. Often, the existing equipment that is available to be used to reduce color might not be suitable for such high temperature operation. It would be desirable if a method could be used over a wider range of temperatures, e.g., lower temperatures or even higher temperatures, so that a wider range of equipment could be used.
Consequently, there remains a strong demand for methods that decolorize amine mixtures to a high degree without suffering from one or more of these significant drawbacks.